President-elect Joe Biden has begun to select candidates for crucial positions in the Cabinet and other senior administration spots. In the running are sitting elected officials. For example, various Democratic Party factions have urged Biden to select his former primary opponents Sens. Elizabeth Warren (D-Mass.), Bernie Sanders (I-Vt.) and Amy Klobuchar (D-Minn) to the Cabinet. Others discussed as potential Biden picks include Reps. Marcia L. Fudge (D-Ohio) and Deb Haaland (D-N.M.).

But should Biden appoint sitting members of Congress, he would risk shrinking the Democratic coalitions he needs to move his agenda through Congress. Here’s why.

States have varying rules for replacing elected officials

States follow very different procedures for replacing elected officials who give up their seats before their terms end. Each presents a different sort of risk for the president’s party.

To replace senators, most states have governors appoint a temporary senator who serves until voters weigh in during a special election. That could be the next regular election for senators, which would be the 2022 midterms. In some states, including Massachusetts and Vermont, that’s a stand-alone special election. Appointing a sitting senator is obviously riskier for presidents if the governor who would appoint the successor comes from the opposition party.

To fill vacant House seats, the U.S. Constitution requires an election, typically held several months after a vacancy occurs. Besides leaving the seat open for a while, the risk here is it flips to the other party in the special election.

What if Biden picks a governor? Michelle Lujan Grisham (D-N.M.) has been mentioned as a possible health and human services secretary. In that case, the new governor would immediately be whoever is next in line in the state’s order of succession. Usually, but not always, this is the lieutenant governor. But in some states, governor and lieutenant governor do not run on a single ticket — and the lieutenant governor may hail from the other party. In that case, a state’s governorship would automatically flip. For instance, in 2009, when President Barack Obama appointed Democratic Arizona Gov. Janet Napolitano to be secretary of homeland security, Arizona’s Republican secretary of state, Jan Brewer, took her place.

Even if someone from the same party takes the vacant office, that successor could lose a subsequent special election or the regular election after that. That’s because appointed officeholders usually lack elected incumbents’ track record of electoral success. Consider Republican Sen. Martha McSally of Arizona, whom that state’s governor appointed to fill the late Sen. John McCain’s seat in 2019. Last month, McSally lost to Democrat Mark Kelly.

Just how risky are these appointments?

We collected data on every Cabinet nominee since 1913. Of these, 42 who were confirmed had previously been incumbent elected officials, including 11 senators, 16 members of the House, 11 governors and four other state or local officials in a partisan office.

Almost 30 percent of all the seats that these nominees vacated had flipped to the other party by the next regular election — either in midterm or presidential years. Not surprisingly, seats which the Cabinet secretary had previously won by a narrow margin were particularly likely to flip. Special elections are idiosyncratic: The opposition party sometimes wins seats considered “safe” for the president’s party.

These flipped seats have several things in common. First, many of those seats flipped under the last nine presidents, reflecting how highly competitive elections have become in our partisan and polarized era. We found only two flipped seats between 1913 and 1969. But since 1969, the parties flipped a dozen seats vacated for the Cabinet — and seven of them since 1992.

Second, flips can have serious policy consequences. For example, after Jeff Sessions vacated his Alabama Senate seat to become President Trump’s first attorney general, Alabama voters replaced him with Democrat Doug Jones. As a result, after McCain died in 2018 — having been the deciding vote to keep the Affordable Care Act — Republicans were unable to try again to repeal the ACA. Brewer, Napolitano’s successor, signed Arizona’s SB1070 immigration law, which required law enforcement to stop and request paperwork from anyone who appeared to be an undocumented immigrant — and which Napolitano would almost certainly have vetoed.

Former lawmakers are a much safer choice

Democrats have very slim margins in the new Congress, which means Biden would risk losing majorities if he elevates incumbents to the Cabinet. Republicans hold a 50-48 advantage in the next Senate; Georgia voters will decide the remaining two seats in early January runoff elections. A Republican-led Senate would complicate Biden’s ability to pass his agenda and could block his Cabinet nominees. In the House, Democrats just barely retained their majority, holding the chamber now by 222-211 (with two seats still undecided). Given differences among factions within the House membership, even a temporary vacancy could put Biden’s agenda at risk.

Presidents who wish to be cautious might wish to select former elected officials instead. For instance, Biden could consider Alabama’s Jones to be attorney general, or former North Dakota senator Heidi Heitkamp as agriculture secretary. These officials have connections in the Senate, which can help ease confirmation. And nominating them would not leave any seats vacant.

Correction: An earlier version of this post incorrectly identified the state which Doug Jones is from in the final paragraph. He is from Alabama. We regret the error.

Jacob Smith (@jacobfhsmith) is assistant research professor of statistical science and political science at Duke University.

Jonathan Spiegler is a PhD candidate in political science at Michigan State University.

Aidan Floyd is an undergraduate student at Duke University.