Senate Parliamentarian Elizabeth MacDonough is the most powerful person in Washington few have heard of.

As the Senate’s chief procedural referee, she will decide how much of President Trump’s agenda can pass without any Democratic votes.

GOP leaders plan to pass legislation reforming the nation’s healthcare system and overhauling the tax code through special “budget ­reconciliation” rules that would prevent a Democratic filibuster.

Republican senators are also looking at scrapping pieces of the 2010 Dodd-Frank Wall Street Reform Act through the use of the same rules.

Their success may largely come down to MacDonough.

“The parliamentarian makes the calls,” said former Sen. Kent Conrad (D-N.D.), who worked with MacDonough in 2010 when Democrats passed a major revision to the Affordable Care Act with a simple majority.

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“She’s able to say what can be voted on with a simple majority versus what requires a supermajority, so this gives the parliamentarian extraordinary influence on the outcome,” Conrad said.

Because they can avoid the filibuster, the budget rules are powerful. But there are major restrictions on their use because of a six-part test in the Senate known as the Byrd Rule, after the late Sen. Robert Byrd (D-W.Va.).

Democrats plan to challenge the GOP by asking MacDonough to make decisions on the most subjective part of the Byrd Rule: whether the budgetary impact of a provision passed under reconciliation is “merely incidental” to its nonbudgetary impact.

If MacDonough rules that a part of a GOP bill has a merely incidental budgetary impact, it would have to be removed from the legislation.

Parts of the ObamaCare bill approved by the House, for example, will almost certainly be ruled out of bounds, senators in both parties have acknowledged.

This could force Republicans to strip out large portions of the House bill, including provisions central to its passage last week. And that could make it harder for the GOP to get a final bill to Trump’s desk.

MacDonough took over as Senate parliamentarian, becoming the first woman to do so, in February 2012, when Democrats controlled the Senate under Majority Leader Harry Reid Harry Mason ReidSenate Republicans signal openness to working with Biden Mellman: The likely voter sham Bottom line MORE (D-Nev.). She was promoted by Secretary of the Senate Nancy Erickson but never talked to Reid about the job.

Hired as an assistant parliamentarian in 1999, when Republicans controlled the chamber, MacDonough served as senior assistant parliamentarian from 2002 to 2012.

She grew up in Maryland before moving to Connecticut at age 13 and later graduated from George Washington University in 1988. She started her career in on Capitol Hill in 1990 as a Senate library aide, later working in the executive clerk’s office and as an assistant editor for the Congressional Record.

She attended Vermont Law School, and after graduating in 1998, worked briefly as an assistant district counsel for the Department of Justice in Newark, N.J., handling immigration cases.

She is respected by Senate Democrats and Republicans alike.

Don Stewart, Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE’s (R-Ky.) deputy chief of staff, praised MacDonough as “a brilliant lawyer, a thorough and fair referee, and a walking encyclopedia of Senate precedent and procedure.

“The Senate is fortunate to have her guidance and advice,” he said.

That respect may be tested in the coming months.

The parliamentarian is supposed to stay above the fray of politics in order to give clear-cut decisions on Senate rules.

But some conservatives have tried to raise questions about MacDonough’s impartiality.

Philip Wegmann, a commentary writer for the Washington Examiner, wrote in March that MacDonough’s “politics are a bit questionable,” asserting that she advised former Vice President Al Gore Albert (Al) Arnold GoreFox's Napolitano: 2000 election will look like 'child's play' compared to 2020 legal battles Who calls an election? Why we need patience and nonpartisanship this time Universal mail-in voting jeopardizes the equal right to vote, but absentee voting protects it MORE in his legal battle with George W. Bush after the 2000 presidential election. He cited a profile published by Vermont Law School, where she received her degree.

MacDonough, however, never played a partisan role after the 2000 election. Instead, she had advised Gore in his capacity at the time as the ex officio president of the Senate during a joint session of Congress while it counted electoral ballots from the 2000 election.

Sen. Ted Cruz Rafael (Ted) Edward CruzLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list 'Parks and Rec' cast members hosting special reunion to raise money for Wisconsin Democrats MORE (R-Texas) has reminded colleagues that the parliamentarian only advises the presiding chair on the Senate’s precedent and the presiding chair can disregard that advice in ruling on a pending question — though this view hasn’t gained any traction within the Senate Republican Conference.

Cruz argued in The Wall Street Journal in March that a full repeal of ­ObamaCare and its mandates should be possible with a simple majority because “the Senate parliamentarian does not ultimately determine what is allowable under reconciliation.”

So far, McConnell hasn’t given any indication that he would do that, but even some Republican senators warn that after McConnell set a new precedent to deny the minority party the power to filibuster Supreme Court nominees, no rule or tradition is safe.

In 2001, then-Senate Majority Leader Trent Lott (R-Miss.) fired Robert Dove, who was serving as the parliamentarian at the time, after Dove ruled against the broader interpretation of the budget rules that Republicans were pushing to pass Bush’s tax plan.

Republicans at the time wanted to pass several tax bills under reconciliation, but Dove advised they could only move one.

MacDonough has weighed in on healthcare and budget reconciliation before.

In 2015, she said a reconciliation vehicle passed by House Republicans to tear down the pillars of ObamaCare needed to be rewritten to accommodate the Senate’s rules.

GOP senators revised language repealing the individual and employer mandates to instead zero out the tax penalties of not complying with them — thereby rendering them toothless while staying in compliance with the Byrd Rule.

Another problem posed by the Byrd Rule is that it requires legislation passed under reconciliation to not increase the deficit beyond the budget window, which is the period of time the Congressional Budget Office (CBO) uses to score the cost of legislation. That window is almost always 10 years.

This could be an issue when Congress moves to tax reform. The Joint Committee on Taxation notified Speaker Paul Ryan Paul Davis RyanAt indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates Peterson faces fight of his career in deep-red Minnesota district MORE (R-Wis.) earlier this year that lowering the corporate tax rate to 20 percent could dramatically shorten the lifespan of the tax cut.

The committee estimated that cutting the corporate rate for only three years would result in a “nonnegligible revenue loss” beyond the 10-year window because companies would carry forward tax credits beyond the next decade.

MacDonough’s power largely comes from being a defender of the Senate as an institution.

“She sets the guardrails for how the Senate can proceed. She’s the interpreter of what they’ve done in the past,” said Sarah Binder, a professor of political science at George Washington University and a senior fellow at the Brookings Institution.

“She’s a huge repository of knowledge and information about what’s been done in the past but also the interpreter of what’s kosher and what’s not kosher,” she added.

Circumventing the parliamentarian would be going down a slippery slope to eliminating the filibuster altogether, said Sam Wice, an attorney and former CBO staffer.

“Reconciliation would be the tip of the iceberg. It’s likely that most of the talk would shift to getting rid of the filibuster,” Wice said.