Reines, a longtime confidant of the former first family, told "Tucker Carlson Tonight" if Clinton believed she would be the best candidate to defeat President Trump she would strongly consider jumping into the race.
"She ran for president because she thought she would be the best president," he said.
"If she thought she had the best odds of beating Donald Trump, I think she would think about it long and hard."
However, Reines rejected the idea Clinton would be at all considering a 2020 run because she is underwhelmed or concerned by the current crop of candidates.
"She's not running because she has any anxiety about the Democratic field -- she really likes a lot of the people running."
He said voters should consider who would be the best person to govern after Trump, in addition to who would be the best politician to defeat him.
While Trump ran in 2016 on a pledge to "Make America Great Again," he now has a concrete record of governance, Reines told Carlson.
"If she were to run, which is unlikely... people would get to decide," he said.
"She might as well run. She might be the best person to beat President Pence --I'm sorry, I meant President Trump."
Prior to Reines' interview, speculation had been growing that Clinton would make a last-minute entry into the 2020 presidential race after reports published Tuesday said members of the Democratic establishment doubted any of the party’s current top candidates can beat Trump next November.
But those Democrats already running said Clinton is doing more harm than good for the party by taking aim at Rep. Tulsi Gabbard, D-Hawaii, in recent remarks.
Clinton in recent weeks has privately stated she would enter the 2020 presidential race if she were certain she could win, The New York Times reported Tuesday. The story, titled “Anxious Democratic Establishment Asks, ‘Is There Anybody Else?’,” said about a half-dozen Democratic donors gathered in New York City questioned whether former front-runner Joe Biden could stand strong against Trump.
Fox News' Danielle Wallace contributed to this report.