Republican presidential candidate Mitt Romney campaigned outside of Philadelphia and Pittsburgh on Monday, a day before Pennsylvania and four other states hold their primary contests.

Romney isn't concerned about the primary, but Pennsylvania will likely be an important swing state in the general election. And Monday also offered a chance to audition a potential running mate: Florida Sen. Marco Rubio.

The historic wave of migration from Mexico to the United States, which over four decades brought 12 million immigrants to the country, has come to a standstill. That's what a new Pew Hispanic Center study released today found.

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ROBERT SIEGEL, HOST:

Lawmakers in the U.S. and Mexico called today for an investigation into Wal-Mart. That's after a damaging story published in The New York Times on Sunday. The Times reported that Wal-Mart's Mexico subsidiary engaged in a massive campaign of bribery, paying more than $24 million to obtain permits to expand its stores across Mexico.

MELISSA BLOCK, HOST:

Journalists make for a pretty tough crowd.

The new issue of The Atlantic asks: Is Facebook Making Us Lonely? The jury's out, though signs point to maybe.

Facebook didn't necessarily make Tanja Hollander lonely, per se, but it did make her curious. It was a little over two years ago when she looked at that number representing "friends," 626 in her case, and started to analyze it.

Six men wearing bright orange prison jumpsuits appeared in a D.C. courtroom today, seeking to overturn their decades-old convictions in a brutal murder by arguing the Justice Department failed to turn over critical evidence that could have helped them assert their innocence.

The Vatican reprimanded America's largest organization of Catholic nuns, the Leadership Conference of Women Religious. The Holy See charged the LCWR with promoting programs with "radical feminist themes" that are incompatible with doctrine on issues ranging from homosexuality to women's ordination.