I'm too lazy to find it right now, but there was an article before the 2016 election campaigns showing that the leader in Iowa (or New Hampshire or one of those early primaries) rarely won the nomination.
That's reassuring at least. And a sample size of n = Ted Cruz is hard to argue with.
I haven't checked out historic New Hampshire Primary results, but here are Iowa Caucuses results courtesy of our friends at Wikipedia. I've highlighted the winners who went on to claim their party's nomination...
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[SIZE=10pt]Democrat[/SIZE]
[SIZE=10pt]1972 - "Uncommitted" (36%), Edmund Muskie (36%), George McGovern (23%), Hubert Humphrey (2%), Eugene McCarthy (1%), Shirley Chisholm (1%), and Henry M. Jackson (1%)[/SIZE]
[SIZE=10pt]1976 - "Uncommitted" (37%), Jimmy Carter (28%) Birch Bayh (13%), Fred R. Harris (10%), Morris Udall (6%), Sargent Shriver (3%), and Henry M. Jackson (1%)[/SIZE]
[SIZE=10pt]1980 - Jimmy Carter (59%)[/SIZE][SIZE=10pt] and Ted Kennedy (31%)[/SIZE]
[SIZE=10pt]1984 - Walter Mondale (49%)[/SIZE][SIZE=10pt], Gary Hart (17%), George McGovern (10%), Alan Cranston (7%), John Glenn (4%), Reubin Askew (3%), and Jesse Jackson (2%)[/SIZE]
[SIZE=10pt]1988 - d!(k Gephardt (31%), Paul Simon (27%), Michael Dukakis (22%), and Bruce Babbitt (6%)[/SIZE]
[SIZE=10pt]1992 - Tom Harkin (76%), "Uncommitted" (12%), Paul Tsongas (4%), Bill Clinton (3%), Bob Kerrey (2%), and Jerry Brown (2%)[/SIZE]
[SIZE=10pt]1996 - Bill Clinton (98%)[/SIZE][SIZE=10pt], "Uncommitted" (1%), and Ralph Nader (1%)[/SIZE]
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2000 - Al Gore (63%), and Bill Bradley (37%)[/SIZE]
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2004 - John Kerry (38%), John Edwards (32%), Howard Dean (18%), d!(k Gephardt (11%), and Dennis Kucinich (1%)[/SIZE]
[SIZE=10pt]2008 - Barack Obama (38%)[/SIZE][SIZE=10pt], John Edwards (30%), Hillary Clinton (29%), Bill Richardson (2%), and Joe Biden (1%)[/SIZE]
[SIZE=10pt]2012 - Barack Obama (98%)[/SIZE][SIZE=10pt] and "Uncommitted" (2%)[/SIZE]
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2016 - Hillary Clinton (49.8%), Bernie Sanders (49.6%), and Martin O'Malley (0.5%)[/SIZE]
[SIZE=10pt]Republican[/SIZE]
[SIZE=10pt]1976 - Gerald Ford (45%)[/SIZE][SIZE=10pt] and Ronald Reagan (43%)[/SIZE]
[SIZE=10pt]1980 - George H. W. Bush (32%), Ronald Reagan (30%), Howard Baker (15%), John Connally (9%), Phil Crane (7%), John B. Anderson (4%), and Bob Dole (2%)[/SIZE]
[SIZE=10pt]1984 - Ronald Reagan (unopposed)[/SIZE]
[SIZE=10pt]1988 - Bob Dole (37%), Pat Robertson (25%), George H. W. Bush (19%), Jack Kemp (11%), and Pete DuPont (7%)[/SIZE]
[SIZE=10pt]1992 - George H. W. Bush (unopposed)[/SIZE]
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1996 - Bob Dole (26%), Pat Buchanan (23%), Lamar Alexander (18%), Steve Forbes (10%), Phil Gramm (9%), Alan Keyes (7%), Richard Lugar (4%), and Morry Taylor (1%)[/SIZE]
[SIZE=10pt]2000 - George W. Bush (41%)[/SIZE][SIZE=10pt], Steve Forbes (31%), Alan Keyes (14%), Gary Bauer (9%), John McCain (5%), and Orrin Hatch (1%)[/SIZE]
[SIZE=10pt]2004 - George W. Bush (unopposed)[/SIZE]
[SIZE=10pt]2008 - Mike Huckabee (34%), Mitt Romney (25%), Fred Thompson (13%), John McCain (13%), Ron Paul (10%), Rudy Giuliani (4%), and Duncan Hunter (1%)[/SIZE]
[SIZE=10pt]2012 - Rick Santorum (25%), Mitt Romney (25%), Ron Paul (21%), Newt Gingrich (13%), Rick Perry (10%), Michele Bachmann (5%), and Jon Huntsman (0.6%)[/SIZE]
[SIZE=10pt]2016 - Ted Cruz (27.7%), Donald Trump (24.3%), Marco Rubio (23.1%), Ben Carson (9.3%), Rand Paul (4.5%), Jeb Bush (2.8%), Carly Fiorina (1.9%), and others (7.3%)[/SIZE]