While vote totals are not irrelevant to presidential elections (especially in the primary phase when trailing candidates quickly find that they lack the financial resources to continue), what ultimately matters is not the popular vote, but winning delegates (for the primaries) and electors (for the general). The delegate math heading into the Iowa Caucuses are different for the two parties for two reasons: 1) the stage at which delegates are bound and 2) the two parties do proportional representation differently.
The Republican math in Iowa is simple. They award twenty-seven delegates (maybe thirty depending upon if the Republican National Convention interprets the binding rules as also applying to the automatic delegates) based on the straw vote taken at the precinct caucuses. A candidate gains a delegate for every 3.7% of the vote that they get. Using the current Real Clear Politics polling average, you would get 8 Trump delegates, 7 Cruz delegates, 4 Rubio delegates, and 2 Carson delegates with the rest of the field getting 1 delegate each. There will probably be some movement on these numbers, but roughly speaking unless the polls are way off, any movement will probably only be a change of one or two delegates.
The Democrats math is much harder. On Monday, the Democrats are only electing delegates to the county convention and each precinct has a different number of delegates to the county convention. In a five-delegate precinct, 50.1% of the vote in that precinct gets you three delegates, but so does 69.1%. There is also the issue of what O’Malley supporters will do. The Democratic Party rules require that a candidate get 15% at the precinct meetings to get any delegates. Barring a very shocking development on Monday night, Governor O’Malley will miss out on that requirement in most precincts and his supporters will have to choose a side between Secretary Clinton and Senator Sanders, a process that will repeat itself at county conventions and the district and state conventions.
Additionally, the number of delegates that each side will get to the county conventions will only give us an idea of how the district and state conventions might play out. It is only at these later stages that delegates will actually be awarded. Even with that caution, unlike the Republicans which only have one state-wide pool of delegates, the Democrats have six pools of delegates (two state-wide and one for each of the four congressional districts).
Beginning with the state-wide pools, the Democrats will proportionately allocate six pledged party leader delegates and nine at-large delegates. These two allocations are done separately. Among the six pledged party leader delegates, it will take 58.4% of the vote at the state convention to get four delegates. Barring a shock on Monday night, the most likely result is a three-three split between Sanders and Clinton. For the at-large pool, getting 50.1% will get a candidate five delegates, but it would take 60.2% to get six delegates. In other words, whomever finishes first is probably only going to have an 8-7 advantage from the statewide delegates.
At the congressional district level, the first (northeastern Iowa) district and the second (southeastern Iowa including the University of Iowa) district will have eight delegates each.; the third (southwestern Iowa including Des Moines) will have seven delegates, and the fourth (northwestern Iowa including Iowa State University) will have six delegates. In the first and second districts, it will take 56.3% of the vote to get a fifth delegate and both candidates are practically assured of three delegates and will probably get four delegates each. Similarly in the fourth district, it will take 58.4% to get a fourth delegate and both candidates are practically assured of two delegates. In the third, the winner will get four delegates, but would have to reach 64.3% to get a fifth. In short, both candidates will probably get at least eleven congressional district delegates with seven up for grabs.
Looking at the overall Democratic delegate math, out of forty-four delegates, both Senator Sanders and Secretary Clinton are almost certain to get 18 delegates, barring something entirely unexpected happening on Monday. The most likely results will be a 24-20, 23-21 (most likely result if the state-wide winner also wins the third district), or 22-22 split ( most likely result if the person who loses state-wide wins the third district).