Investing in Tomorrow's Workforce Act of 2018 This bill allows a business-related tax credit for employers who increase spending on worker training programs compared to their average training expenses for the three previous years. The tax credit is equal to the sum of (1) 40% of the increase for high-demand occupation training expenses, and (2) 20% of the increase for low-demand occupation training expenses. The expenses must be for full-time employees whose compensation does not exceed $82,000 for the year. A "high-demand occupation training expense" is designed to lead to employment in an occupation that is expected to experience not fewer than 20% occupational openings over a specified 10-year period. A "low-demand occupation training expense" is designed to lead to employment in any other occupation.