WASHINGTON — Sen. Marco Rubio expressed skepticism that Donald Trump could allay any of his longstanding concerns about the presumptive GOP nominee’s candidacy, but said he wouldn’t spend the rest of the election attacking him.

“My policy differences and reservations about Donald’s campaign are well-established. I’ve said them often, and I stand by those, those remain. I hope they’ll be addressed but those remain,” he said Tuesday at a forum hosted by the Hudson Institute, a right-leaning foreign policy think tank, in D.C.

“That said, I don’t view myself as a guy who’s going to sit here for the next six months taking shots at him,” he said.

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Rubio said that he “respects and accepts” that Trump is the presumptive GOP nominee but added "that’s not going to change the reservations I have about his campaign, or about some of the policies he’s established.”

His comments — amounting to little more than a lukewarm acceptance of Trump’s likely nomination — came the day after he issued a statement declaring he has “never sought, will not seek and do not want to be considered” for Trump’s vice-president. Rubio’s Tuesday remarks echoed that statement, in which he also said his “concerns with many of [Trump’s] policies remain unchanged.”

But on Tuesday Rubio demurred when asked what Trump could do to reassure him that he’s capable of handling the national security challenges confronting the nation.

“I’m not insisting he change anything — he needs to be true to whoever he is, and if that’s the things he believes in, then he’ll have the chance to make that argument to the American people,” he said.

Still, Rubio promised to be an outspoken advocate for his own views on foreign policy, many of which run counter to the more isolationist doctrine Trump has established over the course of his campaign. Rubio laid out those differences at the very start of his remarks, outlining the national debate over the U.S.’s role abroad and pointing in particular to a “line of thinking that this whole international engagement is a one-way street,” that America gets little in return for its efforts to support and stabilize other nations abroad.

While he said he didn’t “necessarily disagree that sometimes our effort is disproportionate” to what the U.S. receives in return, “we also have to understand we have benefitted from this agreement.”

Rubio made no mention of Trump by name, but seemed to be speaking implicitly of the presumptive GOP nominee, who has called for rethinking NATO and demanding other countries pay the U.S. for its military and humanitarian aid.

“I still think the world without American engagement is a world none of us wants to live with,” he said.

Rubio said it’s important to make the case that “if we are not engaged, the price we pay in the long run is much higher than the price we pay for engagement.”

Rubio hinted that his role this election cycle would be promoting those views by “supporting candidates around the country, especially for federal office, that share those views.”