Sen. Marco RubioMarco Antonio RubioChina's TikTok turns to former lawmakers to help with content moderation policies Hillicon Valley: Warren turns up heat in battle with Facebook | Instagram unveils new data privacy feature | Advocacy group seeks funding to write about Big Tech TikTok adds former lawmakers to help develop content moderation policies MORE (R-Fla.) on Tuesday made his case against presumptive GOP presidential nominee Donald TrumpDonald John TrumpGOP congressman slams Trump over report that U.S. bombed former anti-ISIS coalition headquarters US to restore 'targeted assistance' to Central American countries after migration deal Trump says lawmakers should censure Schiff MORE's foreign policy plans.

Rubio, who suspended his own presidential campaign in March but has yet to endorse Trump, argued against proposals that the U.S. should focus more on domestic problems instead of its global engagement, saying the nation's worldwide presence is important for the economy and national security.

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"It's a lot easier to say let's walk away. It's a lot easier to say, for example, 'Why do we give all this money to NATO and these other people do not?' It's easier to say that than to explain what would happen if you didn't," Rubio said at the Hudson Institute.

"While it is true that there are certainly consequences and complexities presented by our engagement, I still think a world without our engagement is not a world we want to live in.

"If we are not engaged in the world, the price we pay will be much higher in the long run than the price we pay to be engaged," he added.

Trump laid out his "America first" foreign policy model during a speech last month.

He said the country needs to "shake the rust" off its foreign policy, adding that people in the country must know "we're putting the American people first again." 

“Both our friends and our enemies put their countries above ours, and we — while being fair to them — must start doing the same," he said during the speech.

Rubio said Tuesday that the country is improved because of its global engagement, referencing advances in countries such as Japan and South Korea.

"At the end of the day, the products we invent have to be sold somewhere," he said.

On Monday, Rubio said he wasn't interested in being Trump's running mate.

“While Republican voters have chosen Donald Trump as the presumptive GOP nominee, my previously stated reservations and concerns with many of his policies remain unchanged,” he said in a Facebook post.

“He will be best served by a running mate and by surrogates who fully embrace his campaign,” Rubio added. "As such, I have never sought, will not seek and do not want to be considered for vice president.

“Instead, I will focus my attention on representing the people of Florida, retaining a conservative majority in the Senate and electing principled conservatives across the country."

On Tuesday, Trump, in an apparent swipe at Rubio, criticized Republicans who have said they wouldn't be his running mate.

"It is only the people that were never asked to be VP that tell the press that they will not take the position," Trump tweeted.