Washington (CNN) Elliott Management, a major investment firm run by billionaire Republican donor Paul Singer, alerted AT&T Monday of its $3.2 billion stake in the telecommunications giant. In a letter to AT&T's board, Elliott argued the company is underperforming and proposed AT&T sell its non-core businesses and restructure.

President Donald Trump tweeted Monday that the announcement was "great news" and used the opportunity to criticize CNN, whose parent company WarnerMedia is a division of AT&T.

Singer founded Elliott in 1977 and has gained a reputation as an activist investor who accumulates large stakes in public companies in order to force changes in management and strategy. Elliott declined to comment on Trump's tweet about the firm's investment in AT&T. A person familiar with Singer's operations denied the investment was related to politics.

"There's a total firewall between Paul's personal political activities and the firm's investment process," said the person.

In recent decades, Singer has become an influential donor to conservative and Republican causes. He has raised money for GOP presidential nominees George W. Bush, John McCain and Mitt Romney, as well as donated millions to Republican-aligned organizations for both federal and state-level elections, including the Congressional Leadership Fund, the Senate Leadership Fund and the Republican Governors Association.

But with Trump, Singer has been a notable holdout. He endorsed Marco Rubio in November 2015, prompting Trump to attack Singer as someone who "represents amnesty and ... illegal immigration pouring into the country."

In early 2016, Singer funded a super PAC supporting Rubio that ran TV and digital ads sharply critical of Trump. One asked if Republicans "can trust" Trump for holding more liberal positions on health care and economic stimulus. Another documented several of his disparaging comments about women.

After Trump won the nomination, Singer did not donate to his campaign. He did donate $1 million to Trump's inauguration, and in February 2017 met with Trump at the White House. After that meeting, the President claimed Singer had given Trump "his total support."

Since Trump has been in office, Singer has focused his political giving on congressional and state-level Republican campaigns, according to two people familiar with his donations. Singer has not donated to Trump's reelection campaign nor to the super PAC supporting him.

Singer has also not commented publicly on Trump since June 2016. Speaking at the Aspen Ideas Festival, he criticized Trump's proposed trade policies. "And I think if he actually stuck to those policies and gets elected president, it's close to a guarantee of a global depression, widespread global depression," Singer said, according to CNBC

A regular Republican donor

Setting aside his issues with Trump, Singer has otherwise been a traditional Republican donor. He has long been the chairman of the board for the Manhattan Institute, a New York-based conservative think tank largely associated with influencing the policies of Mayor Rudy Giuliani. Singer would later serve as a regional finance chair for Giuliani's failed 2008 presidential campaign, his first significant foray into Republican electoral politics.

Singer has donated to other Washington-based think tanks and issue groups, including the American Enterprise Institute, the Foundation for the Defense of Democracies and the Federalist Society. He has backed both pro-Israel and politically neutral Jewish organizations. The Washington Free Beacon, a conservative online news outlet, is also largely funded by Singer.

The Free Beacon and Singer's involvement attracted attention after the outlet disclosed to congressional investigators in 2017 that it had funded the research firm Fusion GPS during the 2016 GOP primary. The Free Beacon stated it had contracted with the firm "to provide research on multiple candidates in the Republican presidential primary" -- including Trump.

After Trump secured the GOP nomination, the Democratic National Committee and Hillary Clinton's campaign picked up the tab and paid Fusion GPS to continue its opposition research into Trump. Fusion GPS later hired former British spy Christopher Steele to tap into his network of intelligence sources to investigate Trump's ties to Russia. The resulting series of memos Steele wrote subsequently became known as "the dossier."

Those memos correctly identified that the Kremlin was meddling in the 2016 election to help Trump win. They also contained salacious and unverified information about Trump and his connections to Russia and accused the Trump campaign of conspiring with the Russian government to win the presidency. Special counsel Robert Mueller did not find Trump's campaign or his associates conspired with Russia.

A longtime FBI informant, Steele provided some of his work to the FBI, and investigators used some of that information in late 2016 to secure a surveillance warrant against former Trump campaign adviser Carter Page. The Free Beacon maintains that it did not fund or have knowledge of Steele's dossier.

Singer, who has a gay son, has been one of the rare GOP donors to support efforts to legalize same-sex marriage. He in particular took the lead in marshalling support from enough Republican lawmakers in New York to pass the state's Marriage Equality Act in 2011. Singer also funded the legal work that led to the Supreme Court's 2015 decision to guarantee a right for same-sex couples to marry.