Republican presidential candidate Marco Rubio speaks during a campaign event last week in West Des Moines, Iowa. The Florida senator has been looser and more upbeat in the final days before the Iowa caucuses. (Joshua Lott /for The Washington Post)

“Welcome to Muscatine, and thank you for bringing the sun,” a woman joked to Sen. Marco Rubio (R-Fla.) about the pleasant weather here.

“I can’t say I brought the — well, yes, I brought the sun. Vote for me,” he replied, playing along in a crowded brick building on the bank of the Mississippi River. “We’re at that time in the campaign.” The audience erupted in laughter.

Not so long ago, the atmosphere was cloudier for the freshman Florida senator, who is battling for third place with other establishment-friendly Republican presidential candidates and emphasizing the darker side of his message to echo the anger propelling front-runner Donald Trump.

But in Rubio’s final push before the Iowa caucuses Monday, he is looser and more upbeat. His polling position has improved, as reflected in a survey released Saturday that showed him in a reasonably secure third place, and his crowds are bigger and more enthusiastic. Well-known Republicans are joining him. The pack of reporters following him has grown.

He is, in other words, having a bit of a moment.

Whether that will be enough is the big question. Rubio still draws only a few hundred to his events, not thousands like Trump. Many of the voters who come are undecided, and polls suggest Rubio will not win the caucuses. His advisers say his goal is merely to come in third, behind Trump and Sen. Ted Cruz (Tex.).

Rubio’s aides have charted a long path to the nomination that begins with narrowing the field and consolidating the support of other candidates as they drop out. His team is aiming to survive until the March 15 winner-take-all contests and is bracing for a delegate battle with Trump or Cruz. A top-tier showing in Iowa would be a step in that direction. A weak finish could be a devastating blow.

Rubio’s supporters have long believed his natural political talents would lift him to the top of the field. That hasn’t happened, but there are signs that his stump speech — newly calibrated with a generally more optimistic tone than he was sounding earlier this month — is beginning to resonate.

In bars, American Legion halls, colleges and hotel ballrooms across the state, Rubio is describing himself as the more electable alternative to Cruz and Trump, whose antics he has disparaged as a needless distraction.

“Interesting sideshow. Greatest show on Earth. This is not a show. This is serious. We cannot lose this election. We have to turn this country around now,” Rubio said at a pub in West Des Moines on Wednesday.

Rubio was introduced by Sen. Ben Sasse (R-Neb.), who has also campaigned with Cruz and Carly Fiorina. Sasse’s goal: take down Trump.

Rubio has also campaigned with Iowa’s junior senator, Joni Ernst (R). On Saturday night, the state’s senior senator, Charles E. Grassley (R), introduced him.

Republican presidential candidate Marco Rubio released this ad Jan. 26. (Marco Rubio)

Like Sasse, neither is offering an endorsement. But the senators’ message to Iowa voters is a potentially powerful one: We’re comfortable with Rubio’s candidacy, and you can be, too.

The Rubio campaign is airing a 30-minute television special in Iowa this weekend that features town hall footage. The campaign plans to field questions on social media and in telephone calls when the ad is broadcast at different times on different channels.

The challenge facing the 44-year-old son of Cuban immigrants is balancing sunniness — a “New American Century” — with dark warnings of potential storms to come, particularly a win in November by Democratic candidate Hillary Clinton. He repeatedly tells audiences that policies put in place by President Obama, and loathed most by conservatives, would be cemented if the GOP does not field a winning candidate.

“Obamacare will be here to stay,” he said in Des Moines at the event with Ernst.

The hope is to win over voters such as Andy Hall of Des Moines, who has narrowed his choice to Rubio, Cruz or Ben Carson.

“Rubio’s probably the best candidate to go up against a Hillary Clinton or a Bernie Sanders,” Hall said. “He’s not one of those, you know, hard-line right or left guys.”

At his events, Rubio’s campaign team tries to get people to fill out “commit to caucus” cards. The cards are a way to collect contact information so campaign staffers can keep in touch with supporters leading up to Monday.

Rubio is absorbing a torrent of attacks. A pro-Jeb Bush super PAC has pummeled him, using billboards, Web ads and television commercials to criticize his stand on immigration. A woman asked Rubio at a recent town hall to explain the role he played pushing comprehensive reform in 2013 as part of a bipartisan “Gang of Eight,” as a recent pro-Bush ad underscored.

During Thursday's debate in Des Moines, Rubio’s attempts to reconcile his past — and current — hard-line immigration positions with his decision to push that bill were less than clear. He no longer supports comprehensive reform.

Rubio is also fending off aggressive attacks from Cruz. One Cruz TV ad calls Rubio the “Republican Obama.”

The Florida senator’s response: Cruz is afraid of me.

“When a candidate’s being attacked, obviously someone’s worried about you. So, obviously Senator Cruz is worried about my candidacy,” Rubio told reporters in Muscatine. He also raised expectations for Cruz’s campaign in an attempt at gaining a psychological edge.

“Obviously, you know, Ted is the front-runner here. He’s spent a lot of time and money,” Rubio said.

Rubio’s advisers see an overlap between the Republican voters he is courting and the ones Cruz is pursuing, driving months of political combat with the Texas senator that continued during the seventh GOP debate Thursday.

To hear Rubio tell it, Cruz is a flip-flopper who is weak on national security. In Cruz’s version, Rubio is soft on immigration.

When it comes to Trump, Rubio has been less hostile. He has tussled with the mogul over policy, but far less frequently or intensely than he has with Cruz. And in some cases, Rubio has used what sounds like a version of Trump’s “Make America Great Again” slogan at campaign events.

The Rubio campaign has concluded there is little to be gained by hitting Trump hard until the Republican field narrows and a direct fight becomes inevitable. For now, Rubio is drawing mostly implicit contrasts with Trump: “Anger is not a solution. You have to have a plan.”

Rubio’s advisers believe that their most plausible path to the nomination is through a marathon battle to rack up the most delegates for the Republican National Convention in Cleveland in July.

The strategists think that even if Rubio were to finish third in all of the contests from Feb. 1 to March 14, then win Ohio and Florida on March 15, he would be roughly tied for first place in the delegate count.

But right now, Rubio is running a distant third in his home state. And if Trump steamrolls the competition in the February states, he could make it extremely difficult for Rubio or anyone else to emerge as a credible contender in the March contests.

Fundraising is another concern. If Rubio can’t show donors that he can win a state, his war chest could run dry.

At this point, Rubio would welcome such concerns, because it would mean he is alive in the race six weeks from now. Unless he performs well in Iowa, he might never get there. His pitch to still-deciding voters here in the final 48 hours will be especially important.

“I won’t know until Monday,” said Hall, the undecided voter from Des Moines.