– If Sen. Chuck Schumer is going to fulfill his dream of running the Senate, he will likely have to rely on the performance of sometime-rival, sometime-friend and fellow New Yorker Hillary Clinton.

The Democrats' leader-in-waiting needs to pick up five seats to win outright control of the Senate from the Republicans. He's got a national map teed up for a takeover if the party's presidential nominee can inspire a Barack Obama-level performance, whether it's Clinton or Vermont Sen. Bernie Sanders. Schumer could also benefit from a Republican Party at war with itself.

All of that is making Democrats increasingly optimistic about their chances of taking the Senate, particularly if Republicans nominate Donald Trump or Texas Sen. Ted Cruz.

"The Republican leadership knows that's terrible when you have a general election," Schumer said last week about the prospect of a Trump or Cruz nomination.

Five of the Republican seats that Democrats are targeting are presidential battleground states that were won twice by Obama — New Hampshire, Wisconsin, Pennsylvania, Florida and Ohio. Strong enthusiasm for the Republican or Democratic nominee in any of those states could serve as much-needed coattails for Senate contenders.

New Hampshire is a particular bellwether state to watch early on election night to gauge the Democrats' chances. Republican Sen. Kelly Ayotte is facing Gov. Maggie Hassan, Democrats' prize recruit. Both candidates are raising millions and will have no shortage of outside support.

Republicans acknowledge the difficult map and have set a simple goal: Keep the majority. "It's going to be a tough cycle," said Ward Baker, executive director of the National Republican Senatorial Committee.

Some Republican operatives try to brush off the effects of their party's messy presidential primary for now — noting the potential for it to bring an energy boost, while telling their candidates to focus on issues important to their states.

"We can't count on the presidential, we can't count on anyone but ourselves," Baker said.

But some privately — and increasingly publicly — are deeply concerned that a nomination of Trump or Cruz could cost them the Senate and House seats besides. Doug Heye, a former spokesman for the Republican National Committee, declared Thursday in a column on the Independent Journal's opinion website that he wouldn't back Trump if he's the nominee and predicted he'd be a disaster for the party.

Even if Democrats succeed in winning the Senate, the U.S. House looks firmly in the Republicans' grasp, meaning the next president would face a divided Congress — and the prospect of further legislative gridlock.

Meanwhile, Clinton has been staking out centrist positions in recent months — including saying she wouldn't raise taxes on the middle class — a position in sync with Schumer's moderate sensibilities. Schumer has long been ebullient about a Clinton run.

Clinton has already started raising millions in general election money for a coordinated campaign with the Democratic National Committee — money that will help not only the nominee but Democrats up and down the ballot in battleground states.

Republicans, though, note that GOP incumbent senators have early fundraising advantages over their likely rivals, several of whom still have to navigate primaries.