Never has that idiom been truer than right now for Democratic campaigns -- both those for president and those tasked with trying to win back the Senate majority in 2020.

See, candidates like former Texas Rep. Beto O'Rourke, former Colorado Gov. John Hickenlooper and Montana Gov. Steve Bullock are not, well, prospering in their presidential bids at the moment. O'Rourke, who entered the 2020 contest to huge fanfare back in March following his near-upset of Republican Sen. Ted Cruz in Texas, hasn't been able to find any real momentum since then. Hickenlooper's attempt to be a moderate alternative to former Vice President Joe Biden has fizzled. And Bullock got into the race so late that he's struggled to be a factor in Iowa or anywhere.

O'Rourke takes less than 3% in the Real Clear Politics average of all national polling in the presidential race -- which is roughly 10 times the support that either Bullock or Hickenlooper are averaging.

It seems unlikely that any of that trio -- with the possible exception of O'Rourke, although even that looks like a long shot now -- is going to have their desired arc to the top of the presidential field. BUT, all three of them would be absolutely top-tier Senate recruits for Democrats trying to build momentum for a push to the majority next fall. (Senate Democrats need to pick up four seats to retake the majority if they win back the White House and five seats if Trump gets reelected.)

Take Colorado. Freshman Sen. Cory Gardner is widely regarded as among the most vulnerable Republican incumbents in the country. (Colorado is one of two states where a Republican senator is running for reelection in a state Hillary Clinton won in 2016. Maine, where Susan Collins is running for a fifth term, is the other.)

While there are already several credible Democrats in the race, there's no question that Hickenlooper, a two-term governor and two-term Denver mayor, would be a clear favorite for the nomination if he decided to run. While Hickenlooper might not clear the field of other candidates, his candidacy would certainly thin the herd. A poll conducted by the Democratic polling firm of Garin-Hart-Yang Research and released last month showed Hickenlooper with a 50+-point lead over his nearest Democratic rival in a hypothetical Senate primary matchup. And given Gardner's vulnerabilities, Hickenlooper would have to be considered an even-money bet (or better) to be the next senator from the state come 2021.

Which is why, at least according to the New York Times, Hickenlooper is thinking about switching races. "Officials who have been in discussions with the Hickenlooper campaign said Tuesday that the former two-term governor is giving serious consideration to switching to the Senate race but stressed that a final decision has not yet been made," wrote the Times' Reid Epstein on Tuesday night.

Hickenlooper has acknowledged previously that he spoke with Senate Minority Leader Chuck Schumer (New York) about the possibility of a challenge to Gardner. And in an interview with CNN's Ana Cabrera this past weekend, the former governor said that the time was coming when he would need to assess his political future.

"You know, at a certain point I just become stubborn," Hickenlooper said. "And I actually haven't sat down and figured out when that is. I probably need to do that maybe this weekend. But at this point I keep very focused on what I'm doing every day with my team to try and be the next president of the united states. And I don't rule anything out. But I don't -- right now I'm not even thinking about it."

Neither O'Rourke nor Bullock are even that willing to acknowledge the possibility that the Senate might be a very attractive fallback plan if their current presidential aspirations don't work out. But that hasn't stopped plenty of other people from talking about that prospect.

"So Beto, if you're listening: Come home. Drop out of the race for president and come back to Texas to run for senator. The chances of winning the race you're in now are vanishingly small. And Texas needs you."

There's zero indication from O'Rourke that he is even considering that possibility, of course. And like in Colorado, there is already a field of Democrats in Texas running for the chance to take on Republican Sen. John Cornyn. But again, like in Colorado, there's no question that if O'Rourke decided to run for Senate he would be the de facto nominee. After all, this is a candidate who raised more than $80 million and came within a few points of upsetting an incumbent Republican senator in the Lone Star State less than two years ago.

Then there's Bullock -- and Montana. At the moment, Senate Democrats are, roughly, nowhere in the effort to put together a serious challenge against freshman Republican Sen. Steve Daines. Bullock, who is in the middle of his second term as governor, is probably the only Democrat in the GOP-leaning state who could make Daines sweat.

Except, at least so far, Bullock has made clear he's not interested. "His answer on this question has been consistent and it is the same today. Governor Bullock is not running for Senate," a Bullock spokeswoman told Politico in May.

Here's the thing about politics that you have to remember: Circumstances change. And so do minds. While O'Rourke has already qualified for the next Democratic National Committee-sanctioned debate in September, neither Bullock nor Hickenlooper have -- and neither are likely to, either. That makes it very hard to keep making the case to your donors and your staff that you are a viable candidate for the presidential nomination. And while O'Rourke has more support and more money than either Hickenlooper or Bullock, if he doesn't start moving up in the polls in a concerted way sometime soon, he, too, will have to face the hard choice of what his best, most viable political future looks like.

Senate Democrats are content to wait -- for now. The deadline for a candidate to file to run for the US Senate in Texas isn't until December -- and in Colorado and Montana it's not until next year. So there's time.

And Senate Democrats know that if they could convince one or maybe even two of that trio to end their presidential bids and start Senate campaigns, it would make a real difference in the majority math. At the moment, the Cook Political Report, a non-partisan campaign handicapping site -- rates only two of the 22 Republican seats up for re-election in 2020 as "toss-ups." (Those seats are Colorado and Arizona.) A Hickenlooper candidacy would clearly strengthen the party's chances in Colorado, while runs by Bullock or O'Rourke would turn races currently ranked as "solid Republican" by the Cook Report into real contests where both national parties would have to spend significant sums of money to win.

To be clear: Senate bids by Hickenlooper, O'Rourke or Bullock are no guarantee that Democrats would win either those specific states or the broader majority come November 2020. But it would sure improve their chances.