Washington: Just three weeks before the crucial first contest of the primary season in Iowa on February 1, the leading candidates, Republican Donald Trump and Democrat Hillary Clinton, have turned their fire on surging rivals.
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For months Trump insisted he would not criticise his Republican rivals except in retaliation.
He dumped that policy in December when Senator Ted Cruz began to challenge him in Iowa polls.
Trump fired his first volley on December 15, three days after the RealClearPolitics poll average showed Cruz had matched him Iowa.
"I do like Ted Cruz, but not a lot of evangelicals come out of Cuba," he told a town hall meeting in Iowa. "Not a lot come out."
This was Trump testing lines of attack.
The Trump playbook is fairly simple. He appeals to a certain type of voter by pandering to their prejudices. In this case he wanted to see if he could cast Cruz as unchristian or un-American.
Evidently he found these attacks ineffective and now he has settled upon another tactic, suggesting that Cruz, who was born in Canada to an American mother and a Cuban father, is not a "natural born citizen" and is therefore ineligible to serve as president.
"Republicans are going to have to ask themselves the question: 'Do we want a candidate who could be tied up in court for two years?' That'd be a big problem," Trump told The Washington Post last week.
"It'd be a very precarious one for Republicans because he'd be running and the courts may take a long time to make a decision. You don't want to be running and have that kind of thing over your head."
Trump's legal argument is that according to the US constitution the office of president is open only to "natural born citizens". Under common law this is generally understood to include those born to American parents overseas, though this has never been tested in the Supreme Court.
One of the nation's bests known legal scholars, Harvard law professor Laurence Tribe – a man who taught both Cruz and President Barack Obama, and even Supreme Court Chief Justice John Roberts – says the question is unsettled.
Trump doesn't care about the law, he is simply seeking to sow doubt in the minds of voters that Cruz is a viable candidate.
This week Cruz has returned fire. "The Donald seems to be a little bit rattled," he observed on Boston radio.
But his key line of attack has been to smear Trump among sturdy rural conservatives in Iowa as a Yankee carpetbagger.
"I think he may shift in his new rallies to playing New York, New York, because Donald comes from New York and he embodies New York values."
Hillary v Sanders
The same pattern is playing out in the Democratic race, which has been marked by almost otherworldly harmony until now.
Recent polling shows that Mrs Clinton and her social democrat rival Senator Bernie Sanders are now in a dead heat in Iowa and Sanders is leading in New Hampshire, where the second primary will be held on February 9.
Now the knives are out, with both Mrs Clinton and her daughter launching attacks upon Sanders.
During an interview on NBC's breakfast program Today on Wednesday she declared the time had come for her to "draw some contrasts" with Sanders.
"One of the big ones, as you're aware, is on gun safety, where Sanders has been a pretty reliable vote for the gun lobby."
This is in reference to Sanders' record of voting in support of National Rifle Association-backed legislation to give legal immunity to gun manufacturers over wrongful death lawsuits.
Sanders has responded that he fully supports Obama's proposal to make background checks for gun purchases universal.
In campaign events Mrs Clinton has also claimed that Sanders' sweeping package of social programs remain unfunded.
Even Chelsea Clinton has joined the attack, saying during a rally that Sanders would "dismantle Obamacare, dismantle the CHIP program, dismantle Medicare and private insurance."
In fact Sanders' healthcare proposal goes further than Obamacare, creating a single-payer system similar to Australia's.
The attacks reveal much about Mrs Clinton's concerns and understanding of the Democratic electorate.
They show that she is sensitive to Sanders' strength in Iowa and New Hampshire, even though few analysts see him beating her in the long run.
Secondly, they show that for the first time in a generation the Democratic Party is now united in its support for gun control.