President-elect Donald Trump's most powerful adversary in the Senate will be incoming Senate Minority Leader Chuck Schumer. Both men appear to have at least one thing in common: they both rooted for the Democrat when he was up for elections from 1996-2010.

According to the Federal Election Commission's filings, Trump has given Schumer about $9,000 in political donations over a 14-year span. Trump's three oldest children, Donald Jr., Eric and Ivanka have also donated a combined $6,800 to Schumer. Trump’s son-in-law Jared Kushner has given Schumer $4,000.

“The Trump family campaign contributions to Sen. Schumer – like most sizable campaign contributions – are often intended to curry access if not favor with a sitting lawmaker who has some oversight authority over their business interests,” said Craig Holman, a public affairs lobbyist with the government watchdog group Public Citizen. “Schumer has generally shown an independent streak not easily influenced by such contributions, but he is now in a situation of directly negotiating one-on-one with President-elect Trump and the Trump family.”

Holman said that if Schumer were to compromise with Trump those past contributions could give the appearance of "undue influence," adding that the U.S. senator from New York would do himself a great favor by returning the donations.

Asked if he would give back Trump’s donations, Sen. Schumer's communications director Matt House said “it’s a ridiculous question."

"Senator Schumer had no problem standing up to Trump during his campaign and will have no problem doing so in the future,” House added.

Sen. Schumer publicly voiced his support for Hillary Clinton leading up to the election and attended a number of the Democratic candidate's campaign events. He called Trump's rhetoric "empty bravado" during his speech at the DNC.

In April 2011, Trump went on Sean Hannity’s Fox News show to talk about his donations to Democrats when he was considering a run for the Oval Office five years ago, Politico reported. Up until that point, Trump had given the majority of his political donations to Democrats.

“I’ve contributed to Schumer ... I’ve known Schumer for many, many years,” Trump said. “And I have a good relationship with him. The fact is, that I think it is time that maybe we do all get along.”

Trump reinforced his relationship with Schumer on Twitter Sunday, Nov. 20, writing that he has a "good relationship" with the senator and that he's "far smarter" than Sen. Harry Reid.

I have always had a good relationship with Chuck Schumer. He is far smarter than Harry R and has the ability to get things done. Good news! — Donald J. Trump (@realDonaldTrump) November 20, 2016

Starting with the 2012 election cycle, however, Trump exclusively donated to Republicans at the federal level.

Just last month at the Al Smith dinner in New York, Trump jokingly made the remark that Schumer “used to love me when I was a Democrat.”

After Schumer was voted the senate minority leader on Wednesday, he told reporters that he plans to work with Trump when possible.

“When we can agree on issues, then we're going to work with them," Schumer said. "But I've also said to the president-elect on issues where we disagree, you can expect a strong and tough fight."