In the battle for control of the U.S. Senate, there are now at least eight critical contests in which polling shows essentially a dead heat, encouraging Republicans’ hopes that they may yet snag the chamber, which very recently seemed beyond their reach.

Some of the GOP boost is coming from the top of the ticket in the form of Mitt Romney, whose recent surge in the polls seems to be helping Republican candidates across the country.

Democrats still have an edge in their effort to keep control of the Senate, and they may have been helped Tuesday when Republican candidate Richard Mourdock in Indiana suggested that pregnancies resulting from rape are God’s will, possibly damaging his chances to succeed Sen. Richard G. Lugar (R).

But both parties agree that many of the most important races have become more competitive in recent days, and their outcomes harder to predict.

Senate contests in the presidential battlegrounds of Wisconsin and Virginia, where Democrats had leads in polls a few weeks ago, are now essentially even and could be especially influenced if Romney performs well in those states. Polls show Democratic incumbents in Ohio and Florida still ahead, but those races have tightened as Romney has gained ground in the states. And the Senate races in Connecticut and Pennsylvania, long thought to be safe wins for Democrats, have become real contests.

Explore the 2012 Senate map and view unemployment and demographics

The parties and independent groups are now investing heavily in a remarkably long list of states stretching from Maine to Hawaii.

To take control of the Senate, Republicans will need a net gain of four seats if President Obama is reelected, but only three if Romney wins. In that case, Paul Ryan, as Romney’s vice president, would become the Senate’s tiebreaking vote.

A tough road

To be sure, the Republican road to the majority remains a high-wire act requiring GOP candidates to catch almost every break in the last days of the campaign. It begins with the 37 Senate Republicans who are not on the ballot this year and the five GOP nominees in conservative states, including Texas and Wyoming, who are running essentially uncontested races.

Add Nebraska, where polls show Republican Deb Fischer well ahead of Democratic former senator Bob Kerrey, and before the polls even open on Election Day, Republicans can count on having 43 votes in the Senate next year.

Then comes the hard part.

To secure a 51-seat majority, Republicans need to win eight of the nine closest races, including protecting four of their seats where contests are hot. Those include the campaigns in deep-blue Massachusetts, where polls show Democrat Elizabeth Warren now ahead of Republican Sen. Scott Brown, and in Nevada, where Sen. Dean Heller (R) is locked in a tough reelection battle against Rep. Shelley Berkley (D).

But Romney’s rise is giving the GOP new hope. New enthusiasm for the presidential nominee could help in Virginia, Ohio, Florida and Wisconsin.

And Romney romps in Republican presidential strongholds such as Montana, now held by Democratic Sen. Jon Tester, and North Dakota, where Democratic Sen. Kent Conrad is retiring, could also boost his party. Both races have been deadlocked for months and have been relatively immune to movements in the presidential contest. But greater excitement about Romney could translate into increased turnout among GOP voters, which in turn could produce meaningful shifts.

“A rising tide lifts all boats — and the converse is true too,” Sen. Rob Portman (R-Ohio) said with a slight smile on the morning of the final presidential debate. “The more likely scenario is that if Mitt Romney wins, we will take control narrowly.”

Sen. Kelly Ayotte (R-N.H.), another key Romney ally, called it “political reality” that a Romney win would make a GOP Senate takeover more likely.

“If you look at a state like Virginia, some of these other swing states, the Senate seat is likely to flow with a strong performance by Mitt Romney,” she said, referring to the race between former Virginia governors Timothy M. Kaine (D) and George Allen (R).

She added that a good showing by Romney could also benefit former health and human services secretary Tommy Thompson (R) in his campaign against Democrat Tammy Baldwin in Wisconsin.

Democrats’ new urgency

The tighter races have provoked a new level of urgency from Democrats who, while conceding nothing about Obama’s chances, have argued that a Democratic Senate would be a firewall against GOP radicalism in the event of a Romney presidency.

“The Senate has been the bulwark against extremism,” said Sen. John F. Kerry (Mass.). “The Senate is the key to preventing him from putting a policy in place that is dramatically extending the level of unfairness and dramatically increasing America’s economic problems.”

Top donors to the party describe a new flurry of activity and contribution requests from Senate Majority Leader Sen. Harry M. Reid (Nev.) and other leading Democrats.

Sen. Charles E. Schumer (N.Y.), the architect of the current Democratic majority, said that if Romney won and Democrats lost the Senate, it could mean the “dismantling” of the country’s regulatory framework and a rolling back of the Voting Rights Act.

He lamented what Romney’s potential Supreme Court appointments would mean.

“If we have another conservative Supreme Court justice, I think it changes America dramatically for a generation,” said Schumer, who said he has spent much of the past month raising money and campaigning with Democratic incumbents and challengers. He said he speaks daily to five or six Democratic candidates.

“For the hard right, the gold mine is controlling the Supreme Court, and a Republican-majority Senate brings them much further along to doing that,” he said.

Republicans, of course, don’t think the prospect of conservative control is frightening at all.

“A terrible thing might happen if Republicans gain the majority: We might have a budget even! We might even have to debate a budget,” said Sen. John McCain (Ariz.), voicing a common GOP complaint that Democratic leaders in the Senate have not passed a budget in three years.

Regardless of how individual races turn out, it seems clear that neither party will win enough seats to reach a 60-vote filibuster-proof majority. And that means the parties will have to find a way to work together — or continue to live with the gridlock that has plagued Washington in recent years.

“There are not enough votes in the United States Senate for one party to run over the other,” Portman said.

Paul Kane contributed to this report.