Here are 4 key takeaways from Tuesday's primary
Hillary Rodham Clinton, former US Secretary of State (L), and her daughter Chelsea Clinton embrace as they attend the 2015 Meeting of Clinton Global Initiative University at the University of Miami on March 7, 2015 in Coral Gables, Florida (AFP Photo/Joe Raedle)

After six weeks of presidential primaries and caucuses in the United States, Democrat Hillary Clinton and Republican Donald Trump are pulling ahead of the pack.

The following are four key takeaways after Clinton and Trump notched a series of new victories on Tuesday -- and the obstacles faced by their rivals.

1. Hillary Clinton in driver's seat

Clinton, who is hoping to be America's first female president, appears increasingly likely to be the Democratic party's choice to run for the White House in the November 8 election.

"It's pretty much impossible to see how she would not be the nominee," David Hopkins, a professor of political science at Boston College, told AFP.

Clinton now has a considerable lead over her rival Bernie Sanders in terms of delegates, thanks to her wins in at least 18 nominating contests, out of 27.

In the states where Sanders won, he often did so by a small margin, meaning Clinton received nearly as many delegates as he did.

The 68-year-old former secretary of state now has 1,541 delegates -- including the nearly 500 super delegates in her corner -- who will vote for her at the Democratic Party convention in July in Philadelphia, according to a CNN tally. Sanders has roughly half that.

At this rate, she looks hard to catch.

"Bernie Sanders would need to start not only winning states, but winning states by very wide margins in order to make up the numerical deficit that he is now in," Hopkins said.

2. Blacks, Hispanics on Team Clinton

Clinton has won between 70 and 90 percent of the black vote in most states, and two-thirds of Hispanic voters cast ballots for her in Texas and Florida, according to exit polls.

Those margins guarantee her a key advantage in states with significant minority populations. Several of those states -- mainly in the West -- remain on the primary calendar, including California.

Dennis Goldford, a professor of political science at Drake University in Iowa, told AFP that Sanders has trouble connecting with minority voters.

"His message is simply the kind of economic message that you hear day in, day out about millionaires and billionaires and Wall Street and big corporations," Goldford told AFP.

"He's never really done identity politics, when he talks about the concerns of women, or African Americans, or Latinos," he said.

"She has always done identity politics. She is very comfortable doing that."

3. Trump unsinkable -- but not bulletproof

Trump is leading in terms of contests won, votes won and delegates. He is certainly dominating his rivals, despite his controversial statements and a growing anti-Trump campaign.

But to become the Republican party's presidential candidate, he must obtain 1,237 delegates, a majority of the 2,472 delegates in play.

He currently has about half that number, and a more than 200-delegate lead over nearest rival Ted Cruz, the ultra-conservative US senator from Texas.

If Trump cannot reach the magic number before the Republican convention in July, he risks seeing the party's nomination slip through his fingers if delegates end up voting in numerous rounds in what is known as a brokered convention.

"It's almost certain that he will have at least a plurality of the delegates," Hopkins said.

"The question is whether the people who oppose him can stop him from getting the majority before the end of the primaries."

4. What will John Kasich do?

The Ohio governor won the primary in his home state on Tuesday, and thus stays in the race as the third man. His campaign team told AFP that they hope to receive a financial shot in the arm after the departure of Marco Rubio from the race.

By staying in the hunt, Kasich prevents Cruz from becoming the anti-Trump camp's standard-bearer.

His strategy is basically to keep Trump from winning a majority of delegates, and emerging at the convention as the unlikely consensus candidate.

In such a scenario, "the Trump supporters will be absolutely furious," Goldford noted.

"They may well bolt the party or stay home in November," he warned -- not a good option for the Republicans, in the face of the Democratic party's grassroots machine, which works hard to maximize maximum turnout in November.