Mitt Romney’s selection of a vice-presidential running mate, which he is expected to announce soon, could shed valuable light on how he makes big decisions. His choice could shore up what voters view as weaknesses or double down on what they perceive as strengths.

What it won’t do, if history and recent scholarship are any guide, is bring the Republican candidate a critical state or tip the balance in a close election. For all the hyperventilation that accompanies the recruitment of a No. 2, it is unlikely that Rep. Paul Ryan (Wis.), Sen. Rob Portman (Ohio), Louisiana Gov. Bobby Jindal or any other short-lister will give Romney more than an exceedingly modest bump, if any at all.

In a 2010 study of post-election voter surveys from 1968 to 2008, University of California at Irvine political scientists Bernard Grofman and Reuben Kline “find that the net impact of vice-presidential selection is at most one point.” Not trivial, but probably not decisive.

“It makes a very small difference,” said James Campbell, a political science professor at the State University of New York at Buffalo, who has also studied vice presidential choices.

Presidential nominees have won despite choices that were dubious (George H.W. Bush, Dan Quayle) and lost with those well-received (Michael Dukakis, Lloyd Bentsen). While Sarah Palin energized segments of the Republican base for John McCain, she had “very limited success” in attracting women or independents to the ticket, according Brian Brox of Tulane University and Madison Cassels of Penn State in a 2009 study.

“In the final analysis, it seems unlikely that Palin had much of an impact on presidential voting,” they concluded.

Campbell and other researchers agree that 1960 was the last time a vice presidential choice was clearly consequential. John F. Kennedy selected Lyndon B. Johnson expressly to help him in Texas and the South, which he did. More often than not, however, recent presidential candidates have sought qualities other than electoral college clout in a No. 2. Since 1968, 16 of 22 vice presidential candidates were from home states that supported the ticket. But many of those states carried minimal political value or would have been won anyway, such as Delaware (Vice President Biden), Connecticut (Joseph I. Lieberman) or Wyoming (Richard B. Cheney). Candidates more often sought someone who offered more indirect benefits, such as an expertise they lacked, or who appealed to certain segments of voters nationally.

Bill Clinton was widely thought to have strengthened his candidacy with the selection of Al Gore. More than just another Southern baby boomer, Gore addressed holes in his Clinton’s resume. He had Washington experience, a hitch in the Army and no marital troubles. Clinton carried Gore’s Tennessee and its 11 electoral votes, winning the state for the Democrats for the first time since 1976. But the decision to go with Gore paid a larger dividend, according to pollster Stan Greenberg. In focus groups, he was often cited as one of the best things about Clinton’s candidacy.

“Gore’s thermometer ratings are consistently higher than Clinton’s,” Greenberg wrote in a September 1992 memo. He called Gore “an escape hatch for those who want to vote against Bush but have doubts about Clinton.”

Some of Romney’s prospects might tighten races in key states but would not drastically change his position. Public Policy Polling, a Democratically aligned firm that uses automated telephone polls that do not call cellphones, reported last month that in Wisconsin, where President Obama leads Romney 50 percent to 44 percent, a Romney-Ryan ticket would close the gap to 47 percent to 46 percent. In Florida, where PPP has Obama and the former Massachusetts governor in a dead heat, a Romney-Rubio ticket would give Romney a slender two-point edge, still within the margin of error.

In must-have Ohio, according to PPP, 55 percent of voters have no opinion of Portman. Of those who do, 27 percent view him unfavorably, while only 18 percent like him. A Romney-Portman team bumps Obama’s margin in Ohio from three to four points (47 percent to 43 percent), PPP said.

Like other nominees, Romney could decide that Portman’s experience and no-drama style reinforces his own strengths, or that Ryan or Rubio will fire up conservatives.

But the record shows that under any scenario, he’ll have to beat Obama on his own.