Control of Congress before voters Tuesday

Control of Congress hangs in the balance as voters head to the polls Tuesday.

Democrats effectively hold a 51-49 advantage now in the Senate, including independents who caucus with Democrats and the tiebreaker if needed from the vice president.

Republicans currently have control of the House, 220 seats to 212 for the Democrats. There are also three vacancies.

Experts say Democrats are looking at a tough map to retain control of the Senate, but control of the House really is up in the air.

There are 34 Senate seats up for grabs this election, along with all House seats.

Democrats have 23 to 11 seats for the Republicans.

Democrats have too many vulnerable seats to defend in the Senate and don't have the opportunities to gain substitute seats, said Casey Burgat, the Legislative Affairs Program Director at George Washington University.

"Just the map was always so hard" for Democrats, Burgat .

Democratic Sen. Jon Tester is trying to win in deep-red Montana, which Burgat said is looking increasingly difficult. Tester's Republican challenger is Tim Sheehy, a former Navy SEAL.

Tester has held his Senate seat from Montana since 2007, even though his state has voted for the Republican presidential candidate in every election since the mid-1990s.

Charles Hunt, a politics professor at Boise State University who focuses on Congress, said Montana looks like it might be slipping away for Democrats.

Democrats face tough toss-up Senate races in Michigan, Ohio, Pennsylvania and Wisconsin.

Three of those are among just seven swing states in the presidential race.

"We'll see what the trickle-down looks like from the presidential (race). Those are all swing states, huge swing states, outside of (Ohio Sen.) Sherrod Brown," Burgat said. "And then you even throw in Arizona being an open seat. Who knows what will happen there. And then Nevada, they're all going to be tied to the presidential coattails."

Arizona and Nevada are two more .

If voters in Nevada turn out for former President Donald Trump, what will that mean for Democratic incumbent Sen. Jacky Rosen?

Arizona Sen. Kyrsten Sinema, who became an independent who caucuses with Democrats, isn't seeking reelection.

GOP candidate Kari Lake is facing off against Democratic Rep. Ruben Gallego to fill the Senate seat from Arizona.

There's been a decline in split-ticket voting over the years, but Ohio, Nevada and Arizona are all places we could see voters go one direction for president and the other direction for Senate.

For example, a recent showed Trump with a 50% to 49% edge over Vice President Kamala Harris among likely Arizona voters.

But the same poll showed Gallego, the Democratic candidate for Senate, with an eight-point lead over Lake.

The Cook Political Report has , 13 more leaning Democratic, and eight more leaning Republican.

Democrats in competitive House districts have to overcome an and the desire for change.

Republicans in competitive House districts have to overcome the potential distaste moderate voters have for Trump and the .

The most interesting House races will be in the big states of California and New York, Burgat said.

There are 12 seats between those two states – seven in California and five in New York – that fall into a toss-up or leaning category.

"Those are countries by themselves. So, we paint California and New York as already decided. And they are, but only at the presidential level," Burgat said. "But they really have every type of demographic, every type of constituency. And some of these districts are incredibly rural and conservative, even in liberal states."