WASHINGTON—Infusions of ad spending for GOP candidates and persistent voter anxiety over high inflation have brought new momentum to the Republican Party in House and Senate races, analysts say, just as early voting has begun for the midterm elections in many states.

A Democratic lead of about 2 percentage points on the generic ballot—the question of whether voters plan to back a Democrat or Republican for Congress—has been cut by more than half since late September, the FiveThirtyEight average of polling results finds. Democratic leads in many Senate races have declined, according to aggregated polls, and Democratic candidates now trail in surveys in Wisconsin and Nevada, where they were once ahead. Control of both chambers hangs in the balance.

A surge of good news for Democrats in the summer and early fall, as well as a burst of Democratic engagement in the election after the Supreme Court ended federal abortion rights in June, appears to have given way to the factors that traditionally weigh on the president’s party in a midterm election, as many voters tend to sour on the party in power and those backing the party that lost the White House are most eager to vote again.

Activists rallying outside the Supreme Court after it overturned Roe v. Wade in June, eliminating the constitutional right to an abortion.

Photo: nicholas kamm/Agence France-Presse/Getty Images

The bottom line, analysts from both parties say, is Republicans are increasingly likely to gain well over the net five seats needed to retake the majority in the House, which they lost in 2018, while control of the 50-50 Senate could still fall to either party.

“Throughout the summer, there had been quite a few advantages for Democrats, with the dramatic intervention of the abortion decision. It energized younger voters, women and Democrats,” said Democratic pollster Celinda Lake. Now, she said, “you do see some settling down into traditional patterns of off-year elections.”

“Dobbs changed the election and put Democrats back into contention—that’s true,” said Republican pollster Bill McInturff, referring to the high court’s abortion ruling. But about 70% of voters say the country is headed in the wrong direction, he added.

Even as many voters remained concerned about abortion, President Biden is drawing low job-approval ratings, and worries about inflation and the economy remain high.

“There has never been a time when an incumbent party is going to thrive with that set of factors,” Mr. McInturff said.

Since World War II, the president’s party has lost House seats in every midterm election except two, and presidents with low approval ratings fare the worst. In midterms from 1962 through 2018, presidents with job-approval ratings under 50% have lost 39 House seats on average, Mr. McInturff finds, using Gallup data that excludes the Watergate year of 1974. Mr. Biden’s approval rating has been in the low- to mid-40% range in aggregated polls.

Democrats faced a difficult environment heading into this year’s midterms. The party’s legislative priorities in Congress had stalled, and substantially more Democrats in vulnerable House races than Republicans chose not to seek re-election, a factor that usually helps challengers.

Pennsylvania Lt. Gov. John Fetterman, a Democratic candidate for U.S. Senate, had a 2-point lead over celebrity doctor Mehmet Oz in a recent poll, down from a 6-point lead in June.

Photo: Gene J. Puskar/Associated Press

But in August, Democrats navigated a major bill into law that aims to invest in clean energy and reduce healthcare costs. Wariness of new state abortion regulations was credited with helping Democrats win a tossup special election for a House seat from New York and with the unexpected defeat of a referendum in Kansas that would have eliminated abortion rights in the state. Analysts said the Supreme Court abortion ruling drove up voter registration in many states among women, who as a group lean Democratic.

Now, said Nathan Gonzales, editor of the nonpartisan newsletter Inside Elections, “Democratic momentum has stalled. The optimism coming out of the special election wins has waned a little bit.” But at the same time, he said, “we haven’t seen a dramatic shift toward Republicans.”

Several Senate races remain tight, with the two candidates separated by 3 percentage points or less in polling aggregates in four states: Ohio, Nevada, Wisconsin and North Carolina. A poll released Tuesday in a fifth state, Pennsylvania, finds Democrat John Fetterman, the lieutenant governor, with a 2-point lead over Mehmet Oz, the celebrity doctor. Mr. Fetterman had led by 6 points in a June survey by the same pollsters. Georgia’s contest also remains closely watched.

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Many analysts had expected Senate races to tighten as Republicans emerged from contentious primary elections and eventually unified GOP voters behind them, while their allies began spending on TV ads intended to tarnish their Democratic opponents. That appears to be the case in Pennsylvania’s neck-and-neck race, where Mr. Oz and his allies had been outspent 3-to-2 by their Democratic opponents in advertising dollars during July and August, according to AdImpact, which tracks ad spending by campaigns.

Since then, the two sides have been closer to parity in their ad spending, with Mr. Oz and his allies running multiple TV ads that portray Mr. Fetterman as too eager to release dangerous criminals in his role as head of the state Board of Pardons. The Fetterman campaign has said that many inmates were deserving of release, among them those with model records in prison.

Some analysts say that the voting decisions of independents is a top concern.

“A quarter to a third of independents are still undecided, and they moved slightly from leaning slightly Democratic to slightly Republican,” said Ms. Lake. These voters, she said, tend to have lower incomes than core members of either party and are more concerned about the economy and inflation.

Ms. Lake said the Democratic Party’s challenge is to show voters that it has a better economic plan than the GOP, possibly by focusing on the party’s efforts to cut medical costs and protect Social Security and Medicare.

Mr. Biden in recent weeks has released a plan to forgive student loans for many borrowers. On Tuesday, he returned to the issue of legalized abortion, saying he would fast-track a bill to codify abortion rights into law if Democrats maintained control of the Senate and House.

Still, polls show that voters still rate the economy and inflation as the most important issue in deciding which party to support, and they give Mr. Biden low marks for his economic stewardship. In Mr. McInturff’s view, those are substantial problems that will make it hard for Democratic candidates who may be leading, but still shy of a majority, to gain the last few votes they need to win.

“A great candidate can run 8 points ahead of their president,” Mr. McInturff said. ”That’s what fabulous candidates do in a campaign with a flawed opponent. But to do it everywhere, in every campaign, in one night, is really hard. That’s why I think the Senate is in a coin flip.”

Write to Aaron Zitner at aaron.zitner@wsj.com