A young female protester faces down armed police officers at an anti-Vietnam War demonstration outside the 1968 Democratic National Convention in Chicago. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images

Senate Minority Leader Harry Reid, D-Nev., (left) and Majority Leader Mitch McConnell, R-Ky., at a ceremony last month at the U.S. Capitol. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Hillary Clinton is running with all the advantages and challenges of someone who has been in the public eye for more than two decades. Andrew Burton/Getty Images hide caption

itoggle caption Andrew Burton/Getty Images

Sen. Christopher Dodd, D-Conn., left, with then-Senate Minority Leader Tom Daschle at a 1995 news conference on Capitol Hill. Harry Reid took over as leader in 2005 after Daschle unexpectedly lost his re-election. At the time, Reid was unknown to most Americans, but he beat back a challenge Dodd. John Duricka/AP hide caption

itoggle caption John Duricka/AP

Vice President Biden and House Speaker John Boehner wait for President Obama's State of the Union address in front of a joint session of Congress in 2012. As vice president, Biden is also leader of the U.S. Senate, but only gets a vote when senators are evenly divided. Saul Loeb/AP hide caption

itoggle caption Saul Loeb/AP

Former Secretary of State and likely 2016 presidential candidate Hillary Clinton faces the media Tuesday over her use of a private server and email account she used to conduct public business. Richard Drew/AP hide caption

itoggle caption Richard Drew/AP

Protesters filled Wisconsin's state Capitol in Madison on Monday, demonstrating against last weekend's shooting death of Tony Robinson, an unarmed black man. Andy Manis/AP hide caption

itoggle caption Andy Manis/AP

President Obama arrives at the TransCanada Stillwater Pipe Yard in Cushing, Okla., in 2012 after renewed momentum in Congress to approve construction of the northern leg of the Keystone XL pipeline. Pablo Martinez Monsivais/AP hide caption

itoggle caption Pablo Martinez Monsivais/AP

Tea Party supporter William Temple protests against President Obama's health care law outside the Supreme Court in 2012. David Goldman/AP hide caption

itoggle caption David Goldman/AP

The heightened partisanship cemented in congressional districts has created havens for both Democrats and Republicans, whose job security now often depends more on pleasing primary voters than on the high-altitude questions facing the nation at large. Mark Wilson/Getty Images hide caption

itoggle caption Mark Wilson/Getty Images