Sen. Marco Rubio (R-Fla.) is often discussed as a possible 2016 contender, though his past support for a comprehensive immigration bill could hurt an effort to win his party’s nomination. (J. Scott Applewhite/AP)

Sen. Marco Rubio this week may be sending the clearest signals yet that he intends to run for president rather than seek reelection to a second term in 2016.

In a week when the Senate was consumed with a bill to approve construction of the Keystone XL pipeline, Rubio (R-Fla.) was in California raising money for his political action committee and reelection campaign. He was the only Republican senator who did not vote on final passage of the Keystone bill Thursday.

With the West Coast fundraising swing coming on the heels of weekend appearances in California before top GOP donors and in Florida before his strongest supporters, Rubio has firmly reinserted himself into the sprawling roster of GOP presidential hopefuls.

Florida law prohibits Rubio from running for president and for reelection to the Senate at the same time, but until he makes up his mind, he will be forced to position himself for two radically different political contests: a Senate campaign in a swing state that voted twice for Barack Obama and a GOP primary run in which the most conservative elements of the party often dominate.

Top aides say he has not made a final decision. But his attention appears to be focused on a run for president.

“I think he’s certainly seriously exploring the option. And I think he’s very viable,” said Sen. John McCain (R-Ariz.), the 2008 GOP presidential nominee.

Tim Baker, a Republican strategist in Florida, said, “All signs point to him running.”

As the Senate toiled Wednesday over more than a dozen Keystone amendments, Rubio was raising money at a posh Newport Beach hotel. On Thursday, he raised more campaign cash at a breakfast in Beverly Hills.

Democrats in Florida and Washington criticized Rubio for missing work on Capitol Hill, laying the groundwork for future attacks if he runs for reelection.

“It’s not uncommon for presidential candidates to occasionally miss Senate votes,” responded Rubio spokesman Alex Conant. “Senator Rubio hasn’t made any final decisions on 2016, but he is taking the necessary steps to raise the resources he will need should he decide to seek the White House.”

Former Florida House Democratic leader Dan Gelber, who served with Rubio in the state legislature, said the senator’s bigger worry in a reelection campaign would be alienating moderate voters in Florida with the time he has spent appealing to conservative activists who dominate the early presidential nominating states.

Cue the football analogy: “It’s hard to spend all your time in the red zone and then argue you’re between the 40[-yard lines]. Elections are won between the 40s,” Gelber said.

On Sunday, Rubio spoke at a conference near Palm Springs hosted by Freedom Partners, a key group in the vast political network backed by the conservative Koch brothers, a powerful force in GOP politics. He reportedly left a very good impression with the donors in attendance.

Before that, Rubio, who has said he will announce his decision by the spring, huddled with about 300 supporters and donors at his annual PAC retreat in Miami.

The first-term senator’s standing in the 2016 field has been uncertain in recent weeks as his mentor, former Florida governor Jeb Bush, has moved swiftly toward a White House run.

Rubio has said Bush’s maneuvering will not determine whether he runs. But the threat of Bush winning over donors who might otherwise gravitate to Rubio looms over him.

“Ideally, you’d like to say, ‘I’m the only guy from Florida,’ ” Baker said.

Rubio’s past support for a comprehensive immigration bill has also raised doubts about his viability among conservative activists who play a big role in the early states. He has more recently advocated a piecemeal approach.

“He is going to have to be pretty forceful,” conservative activist L. Brent Bozell III said of Rubio’s posture shift on immigration. “He can’t just give a couple of speeches and make the boo-boo go away.”

Rubio’s top advisers say his ability to appeal to a broad cross section of Republicans gives him a leg up on the competitors, many of whom garner intense support from slices of the party but little appeal in other spheres.

“Think about George W. Bush in 2000,” said Rubio pollster Whit Ayres. “He was acceptable to the business Republicans and social conservatives.”

“No one lane is big enough,” Ayres said.

Rubio has been building a national political apparatus in recent years that could quickly morph into a presidential campaign. Anna Rogers, a GOP fundraiser, is the latest addition to his team. She will start at Rubio’s PAC next week.

Through his PAC, Rubio supported Republican Senate candidates such as Joni Ernst in 2014. Ernst won in Iowa and is emerging as an influential power broker in the first-in-the-nation caucus state.

One other signal of his White House ambitions is Rubio’s new book, “American Dreams: Restoring Economic Opportunity for Everyone,” which he has been promoting across the country. Next month, he plans to make a swing through the early-nominating states as part of his book tour. He also intends to travel to Texas and Illinois for more fundraising.

As he moves closer to a run, he has sought to set himself apart from potential foes on foreign policy, emphasizing hawkish positions. The son of Cuban immigrants, Rubio has emerged as arguably the leading critic of President Obama’s decision to normalize relations with Cuba.

While Sen. Rand Paul (R) in Kentucky can be a candidate for dual offices in 2016, Rubio cannot. But he will not fight the law and has said he will choose one race.

But already, speculation is swirling about a third office. If Rubio runs for president and doesn’t win, many Republicans see him as an attractive candidate for governor in 2018.

For now, Rubio is trying to balance the rigors of preparing for a possible White House run with being a senator. Not easy, according to a Republican who has done it before.

“It’s tough, especially early on,” McCain said.