Peggy Noonan on Trump vs. the Clintons, Sanders' new strategy

Republican presidential candidate Donald Trump took to Twitter on Sunday to blast the Republican Party of Virginia for requiring voters to sign an oath affirming they are a member of the party, in order to cast a vote in the primary.

Calling politics a "game of addition," CBS News contributor Peggy Noonan justified Trump's action on "CBS This Morning" on Monday, saying he was "probably correct."

"Politics is a 'game' in which you try to get other people to come in. If you can draw independents and Democrats, good thing," Noonan said. "I've never heard of a stake in modern times asking you before you vote in a primary to swear or state that you are a member of the party. Have the gates be open, have everybody in -- that's the way to do it."

Perhaps another presidential candidate to jump in on the "game of addition" was Democratic presidential candidate Bernie Sanders over the weekend. On CBS' "Face the Nation" on Sunday, the senator explained that he could win over Trump's working class and middle class supporters with his economic policy proposals -- a "smart" move by the senator, according to Noonan.

"He's looking at all the voters and he's thinking, 'I'm a left-wing populist but we have a lot in common with right-wing populists they like Trump,'" Noonan said. "'I'm going to go straight to Trump's guys and say don't just think him, think me. I share some of your views, your antagonisms and your insights, we all hate Wall Street, you know?'"

In the latest of his long string of attacks on Democratic frontrunner Hillary Clinton, Trump went after her husband, Bill Clinton via twitter, bashing his "terrible record" with women. This comes as the Clinton camp announced Monday that the former president would hit the campaign trail on his wife's behalf in New Hampshire.

Despite the intensifying feud between the two candidates, Noonan described the latest clash as another "one of their fights."

"I think this is Mr. Trump being Mr. Trump and Mrs. Clinton being Mrs. Clinton. Interestingly, they're fighting as if they're both the nominees," Noonan said. "It'll be a different fight next week."

But Noonan added that Bill Clinton's campaign trail appearance may not "work in her weight," recalling the 2008 elections, when his statements about then Democratic rival candidate Barack Obama backfired.

"We will see if it's different in 2016, it's eight years later. But if you don't think a Donald Trump is going to go to war with a Bill Clinton... you're just mistaken," Noonan said.