The House inquiry has been plagued by infighting and missteps. The most notable so far was the clandestine meeting to share intelligence between chief investigator, Rep. Devin Nunes (R-Calif.), and the White House he was charged with investigating.

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While the Senate investigative committee has pledged a thorough probe, it’s done little so far. It has held no high-profile hearings. Until very recently, it had no full-time staff, and its few part-time staffers have no investigative experience or expertise with Russia.

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That investigative standstill is worrisome. What’s at stake is the integrity of the U.S. electoral process. But it’s not surprising. The same party controls the presidency and both houses of Congress. And in recent decades, members have shifted their time from committee work to efforts to stay in office, with frequent trips home and ongoing fundraising.

This is why congressional investigations are important

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Throughout American history, from Teapot Dome to the Truman Committee to Watergate, congressional investigations have been a powerful tool to expose wrongdoing, hold the executive branch to account, and prompt policy change.

Indeed, the Senate Intelligence Committee owes its very existence to a congressional investigation. In the 1970s, after Watergate and revelations of controversial surveillance and covert programs, the Senate created the Church Committee to investigate. The committee uncovered systematic abuse in intelligence agencies that shocked the country — and spurred new legislation, including S. Res 400 which created a Senate Select Committee to oversee the intelligence community.

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Although investigations are a powerful tool, Congress hasn’t used them consistently. When the parties are intensely polarized, congressional majorities investigate only when the White House is held by the other party, as our research recently examined.

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Here’s how we did our research

In our research, we identified every congressional investigation of the executive branch from 1898 through 2014. Over slightly more than a century, Congress held nearly 12,000 days of investigative hearings, or 32 years of investigations, an impressive total. However, its willingness to use this power has varied greatly over time. Notably, party loyalties have always threatened to overshadow institutional ones.

Divided we investigate — especially when the parties are polarized

Congress has consistently been more willing to investigate the executive branch when the two are controlled by different parties than when the same party controls both Congress and the White House. And that’s even more true when the parties are especially polarized.

When the parties aren’t as far apart, Congress and the White House can disagree on policy whether the branches are held by the same or different parties. In the past, some committee chairs have disagreed so strongly with a president of their own party that they will investigate.

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For example, Democratic chairmen Martin Dies and Howard Smith launched hostile investigations into New Deal agencies, including the National Labor Relations Board and Office of Price Administration. Similarly, the Democratic chair of the Senate Foreign Relations Committee, J. William Fulbright, spared few punches when investigating the Lyndon B. Johnson administration’s military interventions in the Dominican Republic and Vietnam.

By contrast, when the parties are deeply polarized, the president’s co-partisans have almost no incentive to investigate. Their policy preferences likely align closely with those of the administration. And because most members’ electoral fates are tied to those of their party leader in the White House, they have strong incentives to circle the wagons. Even when the two branches disagree on policy — for example, over the Trump campaign’s alleged ties with Russian President Vladimir Putin — the desire to protect the party wins out.

As a result, when government is divided — different parties control the executive and legislative branches — congressional investigations multiply. And when government is united, congressional investigations stall or disappear.

The figure below shows the average level of investigative activity in a given Congress during three different historical eras. The period from 1898 to 1936 — from the Gilded Age to the depths of the Depression — was, like today, a time of deep partisan polarization. From 1937 to 1980, a cross-partisan “conservative coalition” of Southern Democrats and Republicans held considerable power in both chambers. And since the Reagan era began in 1981, partisan polarization has increased steadily.

It’s no accident, then, that from the late 1930s through the 1970s, Congress routinely investigated alleged executive misconduct, even when government was unified.

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But, in polarized eras (both in the early 20th century and today), when one party held both branches of government, the president’s partisan allies have routinely ignored calls for serious investigations. Right now, investigations are especially few. Committee hearings for any reason — legislation or oversight — have declined amid competing pressures on members’ time.

All this makes the presidency stronger

This institutional failure has far-reaching implications for the balance of power. Congress faces many institutional disadvantages when trying to check the executive. It has 535 members who have little incentive to get along; it operates under complicated procedures that often require super-majorities; and even when Congress can pass laws, the president can veto them.

Investigations offer Congress a way to influence policy despite these limitations. But to check the executive branch, Congress must actually investigate, and do so competently. As the lackadaisical investigation into Russian election hacking has shown so far, Congress often fails this test.

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New hearings may well be in the offing. But committee Republicans know that they examine Trump’s Russia connections at their own political peril. Partisan polarization will likely prevent Congress from keeping the president in check, as the Constitution’s framers envisioned.

Doug Kriner is professor of political science at Boston University.

Eric Schickler is the Jeffrey and Ashley McDermott professor of political science at the University of California at Berkeley.