One way to identify past and future battleground states, decisive voter groups, and key campaign issues is to consider what other observers are writing.
These databases are useful for finding articles from scholarly journals. There are a variety of strategies for choosing search terms; you could represent the previous and upcoming presidential elections by year or the candidates' names, you could represent key campaign issues with words like "economy," "jobs," "healthcare," etc., groups of voters with terms like "Millennials," "African-Americans," "college educated," etc., and U.S. states by name.
The political press is also adept at producing this sorts of analyses and forecasts. You can search for those articles, using similar search terms, on the news, polls, and candidate background page of this guide, using the databases and links in the "media sources" boxes there.
Another way to identify recent battleground states is to look through election results to see which states were close. The links listed in the "Election results and Electoral College maps" box of this guide's general election results page will be helpful. Recent elections could certainly be indicative of which states will be battlegrounds in 2020.
You could also, look through the public opinion polls listed on news, polls, and candidate background page of this guide to see if you can find polling data about voter's opinions of your candidate and President Trump, to gain further insight into which states will be close calls this year.
In addition to analyses of the sort listed above, exit polls from the 2016 election and more recent public opinion polls could also be useful in predicting what campaign issues and voter groups will be important in 2020.
Several sources of public opinion polls are listed on the news, polls, and candidate background page of this guide. In addition to asking voters their preferences about candidates, these polls often also ask them which issues concern them them those most, or how concerned they are about a particular issue.
A link to exit poll data from the 2016 election is on this guide's general election results and exit polls page. Exit polls commonly disaggregate the vote along demographic lines such as race, gender, age, religion, educational background, etc. They sometimes also disaggregate results according to issues of concern, such as "voters who identified the environment as the issue that concerns them most," or "voters who report owning a firearm," etc.
Exit polls are also available for senatorial and gubernatorial elections. If your candidate has previously held or run for Senate or governor, you might be able to find exit polls about those elections and see how the candidate performed among different voter groups.
Another useful resource is The Almanac of American Politics. The almanac profiles current federal office holders, but also provides background information about the political landscape in each state.