Conservatives are hailing the pick, while the left is bracing for significant budget cuts while possibly preparing for a battle over his nomination.

Government spending and the debt wasn't a top issue in the 2016 presidential race, something Mulvaney bemoaned on the House floor earlier this year. On his congressional website, Mulvaney states, "It's disappointing that the discussion about our debt has faded away in the last few years."

Unlike Mulvaney and many conservatives in Congress, Trump did not embrace massive changes to Medicare and Social Security and specifically rejected raising the retirement age for the popular programs.

But Trump's decision to tap Mulvaney as his director of the Office of Management and Budget (OMB) has changed perceptions of how committed the incoming administration is to reducing the nation's debt.

The South Carolina legislator was elected in the Republican wave of 2010, a class that came to power with the help of the Tea Party, a movement created as a direct response to President Obama's stimulus and healthcare reform laws.

A review of the bills Mulvaney has introduced reveals a strong appetite to cutting government spending, especially Medicare and Social Security. He has repeatedly pointed out that both entitlement programs are headed for bankruptcy and must be reformed.

The OMB director is one of the most important jobs in Washington. If confirmed by the Senate, Mulvaney will help shape Trump's budget blueprints and lead the administration's charge on regulatory reform. Most government agencies cannot issue regulations without OMB's approval.

Mulvaney is widely respected by the right and conservatives in Congress on Saturday praised Trump's selection.

Ryan on Saturday lauded Mulvaney as a "conservative reformer from day one..."

House Minority Leader Nancy Pelosi (D-Calif.) ripped Mulvaney, highlighting his support for Ryan's Medicare changes and adding, "We cannot have an OMB director who sees inflicting pain on working families as leverage for his radical agenda."

Rep. Michael Capuano (D-Mass.) said in a tweet that Mulvaney is a "decent man but one of the most hard-right Members I know. Prep for dramatic budget cuts!"

As Trump's budget director, Mulvaney would play a big role on budget negotiations, including raising the debt ceiling — which Congress will have to address in 2017. The 49-year-old father of triplets is adamantly opposed to "clean" debt-ceiling hikes. However, he told CBS's "Face the Nation" last year he could vote for raising the debt ceiling, provided that "action fixes the reason we have a debt ceiling in the first place."

Trump's selection of Mulvaney wasn't a surprise, as the two have been in discussions throughout this month. For Mulvaney to leave Congress, it's likely he is confident that Trump will give him the authority to seek widespread budget and regulatory changes.

Republicans will control the White House and both chambers of Congress in 2017, but revamps to Medicare and/or Social Security would be politically risky. In 2005, President George W. Bush proposed Social Security reform and the plan was so controversial it didn't even get a vote in committee. Democrats won back control of the Congress the following year, mostly because of the Iraq War and GOP ethics controversies.

Mulvaney and Trump share many similar policy positions, having both lambasted Obama's policies on healthcare, immigration, an the Iran nuclear deal.

Meanwhile, some in Congress want the earmark ban imposed by Boehner to be lifted. Mulvaney wasn't a fan of Boehner, but he did extol the Ohio Republican's anti-earmark approach. That issue is likely to come up in Mulvaney's confirmation hearings.

Throughout his congressional career, Mulvaney has immersed himself in budget battles. In a move to embarrass Obama, Mulvaney — spurred on by Ryan — introduced budget plans that mirrored the president's in an effort to put Democrats on the spot. In 2012, Mulvaney's bill on Obama's budget was rejected 414-0. Two years later, it was voted down 413-2. Democrats called the votes a charade and a political stunt. Republicans repeatedly cited those votes on the campaign trail, claiming even Democrats don't back Obama's budgets.

Mulvaney has long backed a balanced budget amendment and helped write the GOP's "Cut, Cap and Balance" measure, which was strongly opposed by the Obama White House. He also has introduced a plan to downsize the government by reducing the federal workforce through attrition. He is a strong proponent of stripping Planned Parenthood of federal funding.

In 2014, he co-wrote an op-ed in The Hill touting bipartisan support for the creation of a commission charged with "eliminating, modifying, or updating regulations that have outlived their usefulness."

A Trump spokesman did not return a request for comment for this article.